Upon stumbling across this thread and reading the OP I thought to myself, "Hmmmm, I've never really thought about it. This could make for an interesting discussion."

Imagine the dissappointment when I found out that it was just a thinly veiled bait & switch to trash genres that end in "core" for not being "metal-ly" enough for the "metal" club. Sub-genre squabbles... ew...

OT:
In addition to what's been said about it being easy to do and having come to be a defining aspect of post 2000 bands, I'd say it comes down to just trying to appeal to people. There's a reason dance, pop, rap etc. appeal to wider audiences, they have rhythm, more explicitly a rhythm you can dance to. Metalheads in a similar manner wanna headbang and it's easier to headbang to something that's more ryhthmic than melodic. I mean that's the reason why drummers concentrate primarily on the kick, snare, and hats when laying down a beat. There's no reason to go all over the kit because then the beat loses it's consistency.

As an experiment take the same rhythm of one of those breakdowns and play it using different notes everytime and see if it has the same effect.

With all that said, it still falls into the same age-old problem of "too much of a good thing is a bad thing." Any technique that get's overused on an album or string of albums gets to be stale and old quickly. Breakdowns are doing just that like harmonized leads, over the moon high vocals, solos with nothing but sweep arpeggios in them; before them.

When did I trash any genre? Oh wait, this is the internet and whenever you have a different belief you're instantly a troll.